Most of the conventional inks are classified into pigment inks and dye inks. Pigment inks have been used in a variety of fields because they are superior to dye inks in light resistance and water resistance and are capable of forming satisfactory recorded images which suffer neither discoloration nor fading. However, the pigment inks at present have the problem of poor ink stability. Specifically, the pigment tends to aggregate with the lapse of time, and this aggregation causes clogging at a pen point or nozzle of an ink jet printer or causes precipitation during ink storage.
On the other hand, dye inks are free from the aggregation and related problems. Hence, the dye inks are extensively investigated and are also utilized in a variety of fields, for example, in non-impact recording methods, which have an advantage of little printing noise. Exceedingly promising among such recording methods is ink-jet recording, by which high-speed recording is possible and recording can be conducted on so-called plain paper without any special treatment for fixation. Various techniques of ink-jet recording have been devised so far; some of these have come into commercial use after being improved, while others are still in the stage of continuous efforts at practical use.
Although either pigment inks or dye inks are usable as inks for such recording techniques, pigment inks have the stability problem and the like problems described above. Hence, dyes are frequently used for avoiding such problems. However, an ink containing a water-soluble dye are unsatisfactory in water resistance, although excellent in stability during storage and ejection. Another problem of these inks is the so-called bronzing phenomenon in which a recorded image formed by using a dye ink becomes light-reflective, as it dries, and comes to have a yellow to brown metallic luster because of the deposition of dye crystals on the paper. Since the recorded image in which the bronzing phenomenon is observed is light-reflective, the recorded image not only has a reduced density but also has a tone considerably different from the desired one.
A known technique for preventing the bronzing phenomenon is to incorporate N-hydroxyethylmorpholine or an alkanolamine, e.g., monoethanolamine, diethanolamine or triethanolamine, into an ink. However, this prior art ink has a drawback because the incorporation of these additives heighten the pH of the ink to 11 or higher even when the additive amount of them is small. Specifically, this high-pH ink has problems concerning ejection, including the corrosion of nozzles which come into contact with the ink, and is also defective in safety in case of accidental touching to the ink during handling. The incorporation of these additives has another problem that it reduces the water resistance of the ink, even though effective in diminishing the bronzing phenomenon. Thus, any truly satisfactory aqueous ink has not been obtained.
An ink for writing utensils, e.g., felt pens, and for other uses is disclosed in JP-A-5-295292, which ink contains a water-soluble dye as a colorant, an amino acid, e.g., L-alanine or glycine, and a resin of an amino acid derivative. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") In this reference, however, any embodiment containing a basic amino acid is not described, and there are also no descriptions therein concerning improvement in the color density and water resistance of recorded images and the prevention of the bronzing phenomenon.